1. Field of the Invention
The invention relates to a stress measurement device and a stress measurement method using a piezoresistive element.
2. Description of the Related Art
A resistance value of a piezoresistive element having a piezoresistive effect changes according to a change in the applied stress. Stress measurement devices have been developed which convert the stress applied to a piezoresistive element into a voltage value using this phenomenon. These stress measurement devices sometimes measure, for example hydraulic pressure in various engines or atmospheric pressure. For example, Japanese Patent Application Publication No. JP-A-8-181331 describes a stress measurement device in which a piezoresistive element is used to form a bridge circuit. In this stress measurement device, a constant voltage supply is used as means for supplying power to the bridge circuit.
When a piezoresistive element is used as a gauge resistance, the piezoresistive element has a positive temperature coefficient with respect to the ambient temperature. That is, the resistance value of the gauge resistance increases according to an increase in ambient temperature. When the piezoresistive element is used in a stress measurement device, a reference voltage increases according to an increase in the resistance value that depends on the ambient temperature. In order to measure the stress by measuring the voltage of the piezoresistive element, it is necessary to compensate for this increase in reference voltage due to the ambient temperature.
As described in JP-A-8-181331, when a piezoresistive element is used to form the bridge circuit, the change in the resistance value is cancelled out by the resistances that form the bridge circuit. Therefore, the output voltage does not easily change according to the ambient temperature. However, with a piezoresistive element in a bridge circuit, at least four terminals and four wires are necessary, which makes it difficult to make the element small. Consequently, when a bridge-type piezoresistive element is used in a stress measurement device, the terminal and circuit configurations of the device become quite complex. When a single gauge-type piezoresistive element that can be formed with two terminals is used, only two terminals and a uniaxial wireare necessary, so a simple circuit can be realized. Using a single gauge-type piezoresistive element that can be formed with two terminals easily enables the device to be small. On the other hand, the single gauge-type piezoresistive element has only single gauge resistance so a change in the resistance value due to the ambient temperature will not be cancelled out in the piezoresistive element. In order to employ a single gauge-type piezoresistive element in a stress measurement device with a wide range of temperatures, it is necessary to compensate for the change in output voltage due to the ambient temperature.